Unmarshal json null relationship data to nil (#65)

When unmarshaling relationship data that is the json literal null to an interface, map, pointer, or slice, set
the value to nil instead of the current behavior, which is to assume the value is already nil and do nothing.
If unmarshaling to any other type, continue to do nothing.

Author: tracli

jsonapi_test.go

@@ -173,6 +173,7 @@ var (
 	articleRelatedInvalidEmptyRelationshipBody  = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{}}}}`
 	articleRelatedInvalidEmptyDataBody          = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{}}}}}`
 	articleRelatedNoOmitEmptyBody               = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":null},"comments":{"data":[]}}}}`
+	articleRelatedLiteralNullData               = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":null},"comments":{"data":null}}}}`
 	articleRelatedAuthorBody                    = `{"data":{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{"id":"1","type":"author"},"links":{"self":"http://example.com/articles/1/relationships/author","related":"http://example.com/articles/1/author"}}}}}`
 	articleRelatedAuthorTwiceBody               = `{"data":[{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}},{"id":"2","type":"articles","attributes":{"title":"B"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}}]}`
 	articleRelatedAuthorTwiceWithIncludeBody    = `{"data":[{"id":"1","type":"articles","attributes":{"title":"A"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}},{"id":"2","type":"articles","attributes":{"title":"B"},"relationships":{"author":{"data":{"id":"1","type":"author"}}}}],"included":[{"id":"1","type":"author","attributes":{"name":"A"}}]}`

reflect.go

@@ -47,3 +47,10 @@ func setFieldValue(fv reflect.Value, v any) {
 	}
 	fv.Set(vv)
 }
+
+func canBeNil(fv reflect.Value) bool {
+	return fv.Kind() == reflect.Interface ||
+		fv.Kind() == reflect.Map ||
+		fv.Kind() == reflect.Pointer ||
+		fv.Kind() == reflect.Slice
+}

unmarshal.go

@@ -293,6 +293,9 @@ func (ro *resourceObject) unmarshalFields(v any, rv reflect.Value, rt reflect.Ty
 			}
 			if !relDocument.hasMany && relDocument.isEmpty() {
 				// ensure struct field is nil for data:null cases only (we want empty slice for data:[])
+				if canBeNil(fv) {
+					fv.Set(reflect.Zero(ft.Type))
+				}
 				continue
 			}
 

unmarshal_test.go

@@ -330,6 +330,19 @@ func TestUnmarshal(t *testing.T) {
 			},
 			expect:      &ArticleRelated{},
 			expectError: ErrEmptyDataObject,
+		}, {
+			description: "*ArticleRelated with nil relationships data",
+			given:       articleRelatedLiteralNullData,
+			do: func(body []byte) (any, error) {
+				a := ArticleRelated{
+					Author:   &Author{ID: "100"},
+					Comments: []*Comment{{ID: "200"}},
+				}
+				err := Unmarshal(body, &a)
+				return &a, err
+			},
+			expect:      &ArticleRelated{ID: "1", Title: "A", Author: nil, Comments: nil},
+			expectError: nil,
 		}, {
 			description: "*ArticleRelated.Author",
 			given:       articleRelatedAuthorBody,